Circadian Rhythms: Our Eyes, Our Rhythms

Another possible disruptor of our circadian clock: aging eyes that admit less light.

Housed in the human brain is a master clock that ties our sleeping, eating, body temperature and other physiological functions to a 24-hour schedule. What’s more, scientists have discovered (as Proto related in “The Timekeepers Within,” Spring 2012) that our genes contain clocks that perform similar functions in each of our organs—to alert the liver, for example, when food will be consumed so its cells can rev up to metabolize and store nutrients. Now a growing body of work is demonstrating how the aging of our eyes can throw circadian rhythms out of sync.

A part of the brain’s hypothalamus called the suprachiasmatic nucleus receives information from photosensitive cells in the retina—not the familiar rods and cones, but other receptors, called photosensitive retinal ganglion cells (pRGCs), discovered by Russell Foster at Oxford in 1999. Upon finding that mice genetically engineered to lack rods and cones still show a predictable pattern of eating and sleeping, Foster began exploring the role that the eye plays in maintaining circadian rhythms.

Researchers have since learned that to keep the master clock ticking optimally, humans need daily exposure to 1,000 to 2,000 lux of light, about the amount they’d get on a very overcast day. When the suprachiasmatic nucleus detects light, it sends signals to the heart, the liver and the pancreas to keep them synchronized to cycles of light and dark. It also regulates hormones and communicates with other organs indirectly by releasing hormones at various times of day. But less light means weaker signals to the SCN. The elderly are especially vulnerable to dysfunctional circadian rhythms when mobility problems keep them indoors. “The light level in a nursing home during the day can be as low as 20 lux, which isn’t enough to regulate the master clock,” Foster says.

As we age, the lens of the eye yellows, cutting down on the amount of light the SCN receives—especially blue light, which has a shorter wavelength. Our eyes transmit the most blue light to retinal cells when we are around 10 years old, according to Patricia Turner, clinical associate professor in ophthalmology at the University of Kansas School of Medicine. On average, transmission of the most critical wavelengths drops 50% by age 45 and 80% by age 75.

Even more light is blocked from getting to the SCN when eyes develop cataracts. Foster studied 335 people who had cataracts removed and found “very clear evidence of significant improvement in sleep quality as more light got into the eye and improved regulation of their master clocks,” he says. What’s more, circadian function improves markedly when colorless intraocular lenses are inserted during cataract surgery. In the late 1980s, some eye researchers advocated for yellow-tinted lenses to block out shorter-spectrum blue light, which they thought damaged the retinas of people who had had cataract surgery and contributed to age-related macular degeneration. Even now, blue-blocking lenses account for about 25% to 30% of intraocular lenses in the United States and about 50% in Japan. The discovery of pRGCs, which are most sensitive to blue light, has underscored the importance of letting in the entire spectrum of visible light.

Light is not just a concern for the aged. Daylight is best for keeping circadian rhythms strong, but most of us don’t get enough of it. The positive effects of light on mood and performance last only 30 minutes without another dose of bright light, regardless of age. “In general,” Turner says, “several hours of light exposure starting early in the morning benefits most people.”

Turns for the Worse
Understanding circadian rhythms may help doctors time treatments for optimum effect.